This invention relates to a method and apparatus for electro-chemically drilling small holes in electrically conductive workpieces.
The smallest holes that can be drilled by conventional electro-chemical machining are about 0.030 inches in diameter, a limitation resulting from the difficulties experienced in producing metal tubes or cathodes that are both adequately insulated and sufficiently stiff below a diameter of this magnitude.
Also known is an electrode assembly, as described, for example, in U.S. Pat. No. 3,467,592 to Andrews having a rod like cathode encased in glass, where the glass extends beyond the rod and is formed at the end of the extension into a nozzle that directed onto a metal workpiece a jet of electrolyte. The glass sheathed rod moves forwardly as machining progresses and the glass nozzle itself enters the hole, said to be as small as 0.005 inches diameter, as the hole deepens. The fragility of these glass nozzle cause great difficulty in their production and storage, and the overall diameter of the hole must be greater than the outside diameter of the tip of the rod.
The method and apparatus described by Andrews have a number of significant limitations. As mentioned above, the electrolyte issued from the electrode is guided by a thin-walled glass tube which must be traversed into the hole being machined in the workpiece. Quite apparently the diameter of the hole must be greater than the outside diameter of the glass tip, and owing to the fragility of the glass tip, practical limitations are placed on how small of a hole such procedures can produce. Another feature of prior processes including that of Andrews is that a flow of electrolyte is used, not a jet, thus when the conductivity of the electrolyte is increased the maching rate increases, but unfortunately the diameter of the hole increases, too. In operation, the flow of electrolyte strikes the bottom of the hole, reverses its direction, and flows as a substantially homogeneous mass up between the outside of the electrode or tube and the hole wall to the exterior. As it flows outwardly, it continues to conduct and consequently etches the hole wall to an undesirably larger size. For most applications, it is desirable to machine only a hole of minimum diameter, especially, for example, in cooling ducts of turbine blades.
It is an object of the present invention to describe a method of electro-chemically machining fine holes in a workpiece, the method obviating the necessity for such fragile apparatus or need of relative movement between the machine and workpiece.
Another object of the present invention is to provide means for drilling or electrochemically machining a plurality of holes in a workpiece using one piece of apparatus and drilling all of the holes simultaneously.
Another object of the present invention is to provide a process for increasing the current conductivity of the electrolyte by heating the electrolyte and thereby increasing the machining rate axially of the jet.